Method of producing synthetic mica



Nov. 21, 1961 NOBUTOSHI DAIMON 3,009,788

METHOD OF PRODUCING SYNTHETIC MICA Filed Oct. 1'7, 1958 b axi Fig. 5

fur En [42F IVobu/a .s/n' 00/07 on 3,009,788 METHQD Q1 PRQDUCINGYNTHETIC MICA Nobntoshi Daimon, 140 Harnsato-cho, Chikusa-ku, Nagoya,Japan Filed Oct. 17, 1958, Ser. No. 767,824 Claims priority, applicationJapan Oct. 25, 1957 8 Claims. (Cl. 233tl4) The present invention relatesbroadly to the art of synthetic mica production, and is moreparticularly concerned with a new and improved method of makingfiuorophlogopite by growing or enlarging mother or seed mica.

Two of the most frequently employed methods of making synthetic mica areto either treat potassium fluosilicate with alumina under pressure andheat, or by melting basic oxides, fluorides and feldspar together. Ithas been found, however, that when the mica melt cools after beingheated by electric current or gas, the melt crystallizes into aggregatesof various sizes. This is believed to be due to inadequate control ofthe cooling rate, disturbances in the temperature gradient, andundercooling of the melt. It is necessary to then separate the crystalsfrom the aggregates by crushing and splitting, however, as a result amajor portion of the mica splits into small fragments and the yield ofcrystals sufficiently large for commercial use is indeed small.

It is accordingly a primary aim of the present invention to provide amethod of producing synthetic mica which avoids the noted disadvantagesof the prior art processes.

Another object of this invention lies in the provision of a method ofmaking mica crystals suitable for practical commercial uses, whichinvolves essentially utilizing relatively small synthetic mica crystalsas the mother or starter for the raw mica components, and which whenmelted and cooled in a crucible under controlled conditions, grow orenlarge into substantial size single or compound mica crystals.

Still another object of this invention is to provide a method of makinga relatively large single mica crystal in a crucible by growing seedcrystals whose a, b and c-axes coincide with those of each of the otherseed crystals, or stated in the alternative, a method of makingrelatively large mica crystals whose crystal axes coincide with those ofthe seed crystals, utilizing seed crystals whose c-axis coincides withthat of each of the other seed crystals, although not necessarilycoinciding with the a and b-axes thereof.

A further object of the present invention lies in the formation of arelatively large mica crystal or compounded crystals whose size andshapes are determined by the contour of the melt crucible.

Other objects and advantages of the present invention will become moreapparent during the course of the following description, particularlywhen taken in connection with the accompanying drawings.

In the drawings, wherein like numerals are employed to designate likeparts throughout the same:

FIGURE 1 is a fragmentary perspective view of a mica sheet crystalemployable in the method of this invention;

FIGURE 2 is a vertical sectional view through a crucible adapted topractice the present method;

FiGURE 3 is a vertical sectional view taken through a somewhat differentform of melt crucible; and

FIGURE 4 is a graphical illustration plotting temperatures alongparticular portions of the crucible against the height thereof.

In accordance with the principles of this invention, crystals are grownor formed from seed crystals without the formation of any new nuclei,with the exception of amass Patented Nov. 21, 1961 melt crucible,preferably together with raw materials. These materials are packed uponthe top of the mica crystals, which are assembled in the same c-axisdirection by standing edgewise with the c-axis of each crystalperpendicular to the temperature gradient. The contents of the crucibleare heated with a predetermined temperature gradient in order tocompletely melt the raw batch while at the same time melting only theupper portion of the seed crystals. When the melt in the crucible iscooled, the unmelted portion of the seed crystals starts to grow withoutthe formation of spontaneously made nuclei. By the described method, asingle crystal or layer of relatively large crystals of definiteorientation are produced.

Referring now to the drawings, there is shown in FIG URE 1 a mica sheet6 belonging to the monoclinic system to which there has been applied thea, b and c-axes thereof. It is to be noted that the c-axis isperpendicular to the horizontal plane of the sheet or crystal 6, andthat the a and b-axes as well as the b and c-axes intersect at rightangles to one another. Further, the a and c-axes lie at an angle ofapproximately 90 one to the other. The angle 5, which is of the order of95 to 100, shows clearly the intersection of the a and c-axes. Appliedalso to the mica sheet 6 are the symbols 001, 010 and 221 designatingthe planes of the crystal as used in crystallographic studies.

Crucibles of various forms and shapes may be employed to practice themethod of this invention, and one crucible of illustrative shape isdesignated in FIGURE 2 by the numeral 5. The crucible as shown isessentially rectangular and located interiorly thereof in uprightrelation upon the bottom wall of the crucible are a plurality of micacrystals 6. As indicated by the legend applied to. FIGURE 2, themultiple crystals 6 are assembled with the c-axis of each crystalhorizontal with the crucible bottom wall, or stated otherwise, thec-axis of each crystal is perpendicular to the side walls of thecrucible. Located upon the crystals 6 in tightly packed or pressedrelation thereon is a predetermined quantity of raw material or micascrap 7 of a composition to be later specifically described. As willalso be noted in detail, the method of this invention is practiced byonly heating a portion of the seed crystals 6, and the numeral 8 in FIG-URE 2 represents the melted section of the starter or mother micacrystals.

The raw materials 7 used in accordance with this invention areaggregates of small crystals or fragments of the seed crystals which areoriginally inserted into the small crystals of synthetic mica orfiuorophlogopite identified by the formula F KMg AlSi O On the otherhand, there may be used aggregates of small crystals or fragments ofsmall crystals of synthetic micas which can be obtained by thereplacement of potassium in fluorophlogopite by cesium, sodium, barium,strontium, calcium or the like, by replacing the magnesium by cobalt,nickel, zinc, iron, manganese or the like, by substituting in place ofaluminum and silicon such elements as cobalt, zinc, iron, manganese,germanium, gallium, boron, beryllium or the like, or as an alternativesubstitution for the aluminum and silicon, there may be used mixtures ofSlog, A1203, KgsiFe, K2CO3, and other components corresponding to thechemical compositions of the fluor-micas.

It may be found upon occasion when melting the batch mixture thatevaporation will occur of a part of the volatile components or adissolution of a part of the crucible material, such as alumina, causinga change in the melt composition which shifts said composition from theexpected ideal chemical composition of fluor-mica. Excess components areaccordingly produced which disturb the regular growth of crystal andproduce crystals of inferior quality. This results in glass or foreigncrystals being deposited between the mica crystal plates, and a markeddeterioration in the flexibility of split mica crystals occurs. Further,the excess components also disturb the formation of single crystals, aswell as retarding the growth rate.

In accordance with the present invention, and for the purpose ofpreventing the formation of deteriorating excess components, thechemical composition of the raw batch mixture may be modified so thatthe resulting melt corresponds essentially with the ideal chemicalcomposition of fluor-mica. To accomplish this purpose, there may beadded a small quantity of volatile components to compensate for theevaporation during melting, and there may further be included a smallamount of those components which might come from the crucible materialto thereby compensate for dissolution during melting.

More specifically, in order to promote the formation of crystals ofsuperior quality, there may be added to the batch mixture in amountsless than 8% by weight fluorides such as PbF ZnF CdF NH F, NH HF K SiFKF and the like. It will be seen upon the practice of the present methodthat the addition agent evaporates and prevents the decomposition ofvolatile components of the batch during the melting thereof.

The formation of crystals of good quality may also be promoted by addingin an amount less than 3% by weight ions such as cobalt, nickel,manganese and the like, which have tetrahedral coordination at hightemperatures and octahedral coordination at lower temperatures. Optimumcrystal formation is also obtained by addition agents which include ionssuch as barium, strontium and the like, whose ionic radii are almostequal to the ionic radius of potassium and can thereby replace potassiumin fluorophlogopite. As indicated, however, particular conditions maydemonstrate that addition agents of the character mentioned will not berequired.

The shape of the seed or mother or starter crystals may be widely variedwithout departure from the principles of this invention. Specifically,large crystals may be placed in the same crystallographic orientation,that is, having the same orientation of the a, b, and c axes. The seedcrystals may also be small crystals assembled in the samecrystallographic orientation, or either large or small crystals placedin the same small c-axis direction. Small crystals may also be assembledin the same c-axis direction and pressed with or wtihout adhesives atroom temperature or at various high temperatures up to the melting pointof the crystals, or the seed crystal may comprise a relatively largesingle crystal.

With regard now to the raw or batch material 7, the batch may be made offluor-mica crystals, or may be a raw material mixture of precisely orabout the chemical composition of fluor-mica. Further, a crystallizedaggregate or aggregates obtained from melt precisely or about the sameas the chemical composition of fluor-mica may be employed, or quenchedglass of the melt may be utilized. In either case, the batch is packedupon the standing edges of the mother or starter crystals in the meltcrucible, in the manner shown in FIGURE 2.

The vessel or crucible containing the seed crystals 6 and raw batch 7 isheated in either an electric or gas furnace, or may be heated internallyby passing an electric current through the batch. The temperaturegradient is set exactly or substantially perpendicular to the c-axis ofthe seed crystals in order to first melt the raw batch and thereafteronly that part of the seed crystals 6 which is in contact with the batch7. The melted part of the seed crystals is identified in FIGURE 2 by thenumeral 8. The crystals whose c-axis coincides with that of the unmeltedseed crystals are then grown by either cooling the entire vessel '5relatively slowly with a fixed temperature gradient, by moving thevessel in the direction of a lower temperature, or by moving the furnaceso that the vessel is located in the cooler part of said furnace.

Substantial investigations have been undertaken of the method hereindisclosed, and specific illustrative procedures which may be followedare set forth in the examples now to follow.

Example 1 Several single crystals of fluorophlogopite were placed asseed crystals on the bottom of a rectangular crucible measuring 1 cm. inwidth, 1 cm. in length and 5 cm. in depth. The crystals were arranged inthe same crystal lographic orientation, that is, in the same orientationof a, b and c-axes with their c-axes being essentially parallel to theplane of the bottom of the vessel. A powdered mass of fiuorophlogopitewas then packed upon the top of the upper ends of the seed crystals, andthe furnace heated to 1400 C. at the level 5 mm. above the upper end ofthe seed crystal layer, providing a temperature gradient of 30 C./cm.around the upper end of said seed crystal layer. The crucible was thenmoved downwardly in the vertical furnace at a rate of descent of 0.7mm./hr., which corresponds to a cooling rate of 2 C./hr. with thetemperature gradient noted. After approximately 50 hours of cooling, thecrucible was taken from the furnace, and it was. observed that theentire batch had grown into a single crystal.

Example 2 Relatively small mica crystals were assembled in the sameorientation of their c-axes, and after mixing with a relatively smallamount of the adhesive agent PbF and hot-pressed with a temperature ofkg./cm. at about 1300 C. The block as thus formed was placed as seedcrystals on the bottom of a rectangular crucible corresponding to thevessel 5 of FIGURE 2, with the c-axis of the block essentially parallelto the plane of the bottom of the crucible. Fluorophlogopite powder wasthen packed in the crucible in contact with the block and the levelabout 5 mm. above the upper end of the block was heated up to about1400* C. and maintained at this temperature with gradient of 60 C./m.After the entire batch and the upper part of the block were melted, thecrucible was slowly lowered at a rate of 0.4 mm./hr. to be therebycooled very slowly at a rate of 2.4 C./hr. After the entire melt hadsolidified, the crucible was removed from the furnace. It was seen thatthe entire charge of batch and the melted portion of the seed crystalblock had grown into relatively large crystals, whose c-axes coincidedwith the c-aXis of the block.

Example 3 A wedge-shaped crucible having a wedge angle of 20, andmeasuring 1 cm. across the top and 5 cm. in height was employed. Theconfiguration of such a crucible is shown in FIGURE 3, the cruciblebeing identified at 11. Several single crystals having the sameorientation as that described in Example 1 above were assembled in wedgeform, and placed in the lower end of a crucible with the sameorientation as in the first example. The crystals of wedge shape aredesignated in FIGURE 3 by the numeral 14, and as will be now noted, araw batch 13 is preferably packed thereupon.

Batch materials were prepared by mixing alumina, magnesia, silica, andpotassium silicium fluoride of the GP. grade. The composition of thebatch was essentially, by Weight, the 11.5% A1 0 27.0% MgO, 30.7% SiOand 25.1% K SiF After the entire batch and a portion of the seed crystalin direct contact therewith had melted, the crucible was withdrawn fromthe furnace to crystallize the melted batch and melted seed crystal intoa single crystal of the same orientation as the crystal layer which wasunmelted. The conditions employed for growing a single crystal includeda temperature at the upper end of the seed crystal of about 1400 C., atemperature gradient of 20 C./cm., and a rate of descent of 1 mm./hr.,providing a cooling rate of 2 C./hr. After solidification of the entiremelt,

the rate of withdrawal was increased from 0.7 rum/hr. to 50 mm./hr.

Example 4 A wedge-shaped crucible of the same character described in theforegoing example was employed, and an essential departure in theprocedure followed was to eliminate the use of raw batch. The entirecharge used was a wedge-shaped block, assembled in the same mannor asthe seed crystal block employed in Example 3. The wedge block was packedin the crucible with its caxis perpendicular to one of the wedge wallsof the crucible and a substantial portion of the block with theexception of the apex thereof was melted at a temperature of about 1400fC. The unmelted portion or tip served as seed crystals in cooling. Inthis procedure a temperature gradient of C./cm. was employed, and a rateof descent of 0.2 mm./hr. utilized to provide a cooling rate of 04C./hr. The crucible and furnace were then cooled by cutting ofi the heatsupply.

It will now be seen that a single crystal or large crystals may beformed having sizes and shapes determined by the contour of the meltcrucible, by utilizing synthetic mica crystals as the starter, thecrystals being arranged so that their c-axes coincide. While it wasbrought out that the crucibles are removed slowly from the heat zone,essentially the same results may be obtained with a fixed crucible and amovable furnace. Also, essentially the same results are obtained bycooling the temperature of the crucible placed in the furnace, which hasa predetermined temperature gradient. Gas or electrical furnaces may beemployed, and horizontal furnaces utilized as a substitute for thevertical type. When using a horizontal furnace, the c-axis of the seedcrystals is placed perpendicular to the axis of the furnace.

The synthetic mica herein produced is characterized by a highertemperature stability than natural mica, and thereby finds importantapplications such as in various electronic tubes the high operatingtemperatures of which cause fatigue and short life in natural mica. Thesynthetic mica as made by the processes herein disclosed is of a morepure composition than natural mica since all elements can be morereadily controlled. The replacement of the (OH) radical from naturalmica with P to produce synthetic mica of course results in the advantagethat synthetic mica can withstand temperatures in excess of 1000 C.,since the F is not readily vaporized. Accordingly, if the (OH) radicalis replaced by a fluoride, natural mica scrap can be employed as thebatch material over synthetic seed mica. As earlier disclosed, these newand improved results are obtained by arranging synthetic mica crystalswith their c-axes perpendicular to the temperature gradient as shown inFIGURE 4 of the drawings.

It is to be understood that various other modifications may be eifectedin the compositions, structures and procedures herein utilized withoutdeparting from the novel concepts of the present invention.

I claim as my invention:

1. A method of producing fluor-mica, which comprises arranging at leastone fluor-mica seed crystal on edge in a heating vessel with the c-axisof the crystal substantially perpendicular to the temperature gradientwhich is perpendicular to the bottom of the vessel, 10- cating upon theexposed upper edge of the crystal a raw material having a chemicalcomposition corresponding to that of fluor-mica, applying a controlledheat and melting the raw material and the upper portion only of thecrystal to form a crystal the c-axis of which is oriented with thec-axis of the unmelted portion of the seed crystal.

2. A method of producing fluor-mica, which comprises arranging aplurality of fluor-mica seed crystals on edge in a heating vessel withthe small c-axes of the crystals substantially perpendicular to thetemperature gradient which is perpendicular to the bottom of the vessel,and with the a, b and c-axis of each crystal oriented with 6 the sameaxes of the other crystals, locating upon the exposed upper edges of the"crystals an aggregate of relatively small fragments of iluo-r-mica,applying a controlled heat and melting the aggregate and the upperportion only of the crystals, and cooling the melted aggregate andmelted portion of the crystals to form at least one single crystal thec-axis of which is oriented with the c-axes of the unmelted portion ofthe seed crystals.

3. A method of producing fluor-m-ica, which comprises assembling aplurality of floor-mica seed crystals with their c-axes aligned and witheach seed crystal adhesively secured to another crystal to form a shapedblock, locat ing the crystal block on edge in a heating vessel with thec-axcs of the crystals substantially perpendicular to the temperaturegradient which is perpendicular to the bottom of the vessel, locatingupon the exposed upper edges of the crystals an aggregate of relativelysmall fragments of fluor-mica, applying a controlled heat and meltingthe aggregate and the upper portion only of the seed crystals, andcooling the melted aggregate and melted portion of the crystals to format least one single crystal c-axis of which is oriented with the c-axesof the 1mmelted portion of the seed crystals.

4. A method of producing fluor-mica, which comprises arranging aplurality of fluor-mica crystals on edge in a heating vessel with thec-axes of the crystals substantially perpendicular to the temperaturegradient which is perpendicular to the bottom of the vessel, locatingupon the exposed upper edges of the crystals a raw batch mixturecomprising synthetic mica aggregate and an addition agent selected fromthe group consisting of NH F, NH F ZnF CdF PbF- and K SiF applying acontrolled heat and melting the aggregate and the upper portion only ofthe seed crystals, and cooling the melted aggregate and melted portionof the crystals to form at least one single crystal the c-axis of whichis oriented with the c-axes of the unmelted portion of the seedcrystals.

5. A method of producing fiuorophlogopite, which comprises arranging aplurality of fluorophlogopite seed crystals on edge in a heating vesselwith the c-axes of the crystals substantially perpendicular to thetemperature gradient which is perpendicular to the bottom of the vessel,locating upon the exposed upper edges of the crystals a raw batchmixture comprisingfluorophlogopite aggregate and an addition agentselected from the group consisting of a compound of cobalt, manganeseand nickel and whose coordination number changes 4-6 times duringcrystallization, applying a controlled heat and melting the aggregateand the upper portion only of the seed crystals, and cooling the meltedaggregate and melted portion of the crystals to form at least one singlecrystal the c-axis of which is oriented with the c-axes of the unmeltedportion of the seed crystals.

6. A method of producing fluorophlogopite, which comprises arranging aplurality of fluorophlogopite seed crystals on edge in a heating vesselwith the c-axes of the crystals substantially perpendicular to thetemperature gradient which is perpendicular to the bottom of the vessel,locating upon the exposed upper edges of the crystals a raw materialhaving a chemical composition corresponding to that of fluor-mica andnot more than 3% by weight of an addition agent selected from the groupconsisting of a compound of cobalt, manganese and nickel and whosecoordination number changm 4-6 times during crystallization, applying acontrolled heat and melting the raw material and the upper portion onlyof the seed crystals, and cooling the melted raw material and meltedportion of the crystals to form at least one single crystal the c-axisof which is oriented with the c-axes of the unmelted portion of the seedcrystals.

7. A method of producing fluor-mica, which comprises arranging aplurality of fluor-mica seed crystals on edge in a heating vessel withthe c-axes of the crystals substanaggregate and melted portion of thecrystals to form at 10 least one single crystal the c-aXis of which isoriented with the c-axes of the unmelted portion of the seed crystals.

8. A method of producing fluor-mica as defined in claim 1, in which thetemperature gradient is parallel to the horizontal plane of the heatingvessel.

References Cited in the file of this patent UNITED STATES PATENTS2,516,983 Hatch Aug. 1, 1950 2,645,060 Waggoner M July 14, 19532,675,853 Hatch et a1 Apr. 20, 1954 OTHER REFERENCES Hatch: SyntheticMica Investigation, Bureau of Mines Report of Investigation 5337, June1957, pages 24-45.

Kendall: Proceedings of International Congress of Pure and AppliedChemistry, pages 167 to 170, 1947.

1. A METHOD OF PRODUCING FLOUR-MICA, WHICH COMPRISES ARRANGING AT LEASTONE FLOUR-MICA SEED CRYSTAL ON EDGE IN A HEATING VESSEL WITH THE C-AXISOF THE CRYSTAL SUBSTANTIALLY PERPENDICULAR TO THE TEMPERATURE GRAADIENTWHICH IS PERPENDICULAR TO THE BOTTOM OF THE VESSEL, LOCATING UPON THEEXPOSED UPPER EDGE OF THE CRYSTAL A RAW